linux-kernel - Re: The performance and behaviour of the anti-fragmentation related patches

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Message-ID: <45EA27BD.7080909@redhat.com>
Date:	Sat, 03 Mar 2007 20:58:21 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Nick Piggin <npiggin@...e.de>,
	Christoph Lameter <clameter@...r.sgi.com>,
	Mel Gorman <mel@...net.ie>, mingo@...e.hu,
	jschopp@...tin.ibm.com, arjan@...radead.org,
	torvalds@...ux-foundation.org, mbligh@...igh.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

Andrew Morton wrote:
> On Sat, 03 Mar 2007 20:26:15 -0500 Rik van Riel <riel@...hat.com> wrote:
>> Nick Piggin wrote:
>>
>>> Different issue, isn't it? Rik wants to be smarter in figuring out which
>>> pages to throw away. More work per page == worse for you.
>> Being smarter about figuring out which pages to evict does
>> not equate to spending more work.  One big component is
>> sorting the pages beforehand, so we do not end up scanning
>> through (and randomizing the LRU order of) anonymous pages
>> when we do not want to, or cannot, evict them anyway.
>>
> 
> My gut feel is that we could afford to expend a lot more cycles-per-page
> doing stuff to avoid IO than we presently do.

In general, yes.

In the specific "128GB RAM, 90GB anon/shm/... and 2GB swap" case, no :)

> At least, reclaim normally just doesn't figure in system CPU time, except
> for when it's gone completely stupid.
> 
> It could well be that we sleep too much in there though.

It's all about minimizing IO, I suspect.

Not just the total amount of IO though, also the amount of
pageout IO that's in flight at once, so we do not introduce
stupidly high latencies.

-- 
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.
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